Surface acoustic wave (SAW) devices are critical components in wireless communication systems. SAW devices have been successfully employed for various commercial applications, such as intermediate-frequency (IF) filters for televisions and radio-frequency (RF) filters for mobile telecommunications. Accompanying the remarkable progress of the telecommunications field focusing primarily on cellular telephones and other mobile communications, the demand for surface acoustic wave elements, e.g., tuners capable of receiving broadcast satellite (BS), commercial satellite (CS) broadcasts and radar systems, is increasing rapidly. Therefore, high-performance, high-frequency SAW devices with low insertion loss are required.
A SAW device typically consists of interdigital transducer (IDT) metal electrodes together with a piezoelectric medium for wave generation and propagation. The piezoelectric effect is used to convert electrical energy into acoustic wave energy and vice versa. When an electrical signal is applied to the interdigital transducer electrodes, which are deposited on the piezoelectric member, mechanical stress can be induced followed by the development of a geometric deformation on the piezoelectric member that generates a SAW, which in turn is propagated along the piezoelectric member and received by the other interdigital transducer electrodes as an electrical signal. Information carried by the SAW along the surface of a crystal substrate can be processed during propagation. The signal center frequency (f0) of a SAW device is determined by the velocity of the acoustic wave (υφ) and the spatial period (wavelength λ) of the interdigital transducers (IDT), so the device has a bandpass characteristic with a center frequency (f0), which is expressed as fo=υφ/λ.
The increase in the amount of information transmission with high bit-rate requires SAW devices operating in high frequencies such as the microwave range. Much effort has been made in the development of higher frequency SAW devices in the GHz range. Increasing the center frequency of SAW devices requires high acoustic wave velocity and/or small wave length (the small IDT spatial period). The wavelength λ is generally in the range from hundreds of nanometers to micrometers due to limitations in fabrication technique in IDTs. Reduction of the electrode size also suffers from problems such as reliability, power durability and fabrication margin in the manufacturing processing. Therefore a major effort to increase the center frequency was made through increasing the velocity of surface acoustic waves. The SAW velocity depends mainly on the nature of the medium wherein it transfers. Thus searching for materials with high acoustic velocity is essential for achieving high-frequency SAW devices.
So far, piezoelectric materials used for the SAW device include bulk single crystals, such as lithium niobate, lithium tantalite and quartz, and thin films such as ZnO and AlN films that are deposited on substrates. The single-crystalline piezoelectric bulk LiNbO3 yields a propagation velocity of 3,500 to 4,000 m/s, and LiTaO3 of 3,300 to 3,400 m/s. If SAW devices using these piezoelectric materials are expected to work in high frequency range, e.g., 10 GHz, the spatial period (wavelength λ) of the IDT must be reduced to less than several hundreds nanometers. To meet the demand for high-frequency and wide-band applications, diamond layer based structures have been employed.
Diamond, the hardest material in the world, is a very promising material for high-frequency SAW filters because it has the highest acoustic velocity among all materials. In addition to the highest Young's modulus, diamond has the highest thermal conductivity as well, which may provide an advantage for high power handling of diamond-based SAW devices. However, diamond itself does not exhibit a piezoelectric effect, therefore to excite SAW in diamond, a piezoelectric layer is required for electromechanical conversion. Polycrystalline diamond films have been successfully synthesized on various substrates by chemical vapor deposition (CVD). Combined with piezoelectric thin films, such as ZnO or AlN, high-frequency SAW devices operating in the gigahertz range based on polycrystalline CVD diamond films have been achieved. Moreover, by using ZnO/nanodiamond/Si and AlN/nanodiamond/Si layered structures, the performance of as-deposited nanodiamond films in the SAW devices was investigated. The high-frequency characterization showed that nanodiamond presents a high surface acoustic velocity similar to that of polycrystalline diamond. So far, the SAW filters of SiO2/ZnO/diamond/Si structure with center frequencies from 2.48 to 5.0 GHz was fabricated, and the insertion loss of such SAW filters was reduced to the range from 1.3 to 3.2 dB. The IDT/AlN/Diamond SAW devices have shown an operating frequency at approximately 2.5 GHz.
For diamond-based SAW devices, the choice of the piezoelectric material is another important consideration, because the phase velocity is determined by the elastic constants of both underlying diamond and top piezoelectric layer. A material with a similar phase velocity to that of diamond will be preferred to minimize the velocity dispersion at diamond/piezoelectric layer interface, and thus to enhance the electromechanical coupling coefficient of the layered structure of SAW devices. Among the piezoelectric materials, cBN (isostructural to diamond), with hardness and elastic modulus next only to diamond, leading to very high sound propagation velocities in the bulk as well as in the surface, would be a very promising candidate for fabricating the new high-frequency, high-performance SAW devices. In order to improve the operating frequency of SAW devices and meet the demands for high frequency and wide band applications, a new SAW device based on cBN/diamond composite structure is presented in the present invention.